This invention relates to optical amplifiers, in particular to optical amplifiers for handling wavelength multiplexed signals, and to optical transmission systems incorporating such optical amplifiers. Such a system may for instance comprise a wavelength division multiplexed signal transmitter 10 (FIG. 1) optically coupled with an optical receiver 11 by way of a transmission path 12 that includes a cascade of amplifiers 13 optically in series.
In a transmission system that does not employ wavelength multiplexing, a single optical channel can be transmitted through a cascade of quite a number of optical amplifiers before its form has degenerated to the point at which full regeneration is required. The spectral gain characteristic of a typical optical amplifier, such as an erbium doped fibre amplifier, is wide enough to support amplification of several wavelength multiplexed signal channels. However the spectral gain characteristic is not entirely uniform over its full usable spectral range. Thus an erbium doped fibre amplifier may show a significant peak in its gain characteristic near the short wavelength end of that characteristic. The use of filters to flatten such characteristics is known, but is still liable to leave the characteristic with residual undulations. Therefore, if a wavelength multiplexed set of signal channels is transmitted through a concatenation of such amplifiers, those channels registering with any slight troughs in the gain characteristic will not be amplified as much as other channels registering with any slight peaks. Quite a small difference in gain per amplifier can become quite significant in a long concatenation of amplifiers because the difference is multiplied by the number of amplifiers. Thus for instance if there is 0.5 dB difference in gain per amplifier between a channel at wavelength .lambda..sub.1 and a channel at wavelength .lambda..sub.2 then, if both channels were launched with equal power, after passage through a concatenation of 20 such amplifiers, those channels would differ in power by 10 dB , thereby bringing the weaker channel much closer to, or even beneath, the noise floor. An additional problem is associated with the fact that the ratio of the gain of such an amplifier at two wavelengths .lambda..sub.1 and .lambda..sub.2 is functionally dependent upon the gain provided by that amplifier, and so if a particular amplifier is adjusted to provide equal gain for two wavelength multiplexed channels at a particular drive level, for instance by selective attenuation of the wavelength of one of the channels then that equality will not be maintained if the drive level has subsequently to be increased (or reduced).
One way of overcoming, or at least ameliorating, these problems is to arrange, in at least some of the amplifiers, for the channels to be demultiplexed, separately amplified, and then remultiplexed. Such an arrangement is described in GB 2 276 787, to which attention is directed.